Practice motion in one and two dimensions, including vectors and projectile motion. Quiz one topic at a time or take the whole unit.
The idea each topic turns on, and the mistake students make most often.
Key ideaDisplacement and velocity have direction; distance and speed do not. Average velocity is displacement divided by time, not distance divided by time.
Common mistakeUsing distance traveled when the question asks for displacement, or treating speed and velocity as the same thing when the object changes direction.
Key ideaThe slope of a position–time graph is velocity: its sign gives the direction, and its steepness gives the speed.
Common mistakeReading the height of the graph as the velocity. A high point means far from the origin, not fast.
Key ideaIn free fall the acceleration is g downward the whole time, including at the top of the path where the velocity is momentarily zero.
Common mistakeSaying the acceleration is zero at the highest point. Only the velocity is zero there.
Key ideaOn a velocity–time graph, the slope is acceleration and the area between the graph and the time axis is displacement (negative below the axis).
Common mistakeTreating a negative velocity or a negative acceleration as slowing down. An object slows down only when velocity and acceleration have opposite signs.
Key ideaList the known quantities (Δx, v₀, v, a, t) with signs, pick the one kinematic equation that leaves out the quantity you don’t know and don’t need, and keep one sign convention throughout.
Common mistakeMixing directions, such as writing g as +9.8 m/s² while calling upward positive, or using the constant-acceleration equations when the acceleration changes.
Key ideaVelocities add as vectors: the velocity relative to the ground is the velocity relative to the moving frame plus the frame’s velocity relative to the ground.
Common mistakeAdding speeds as plain numbers when the velocities are at an angle, or subtracting in the wrong order.
Key ideaBreak vectors into perpendicular components (if θ is measured from the x-axis, Ax = A cos θ and Ay = A sin θ), add the components separately, and then recombine.
Common mistakeAdding magnitudes directly, or using sine and cosine the wrong way round because the angle was measured from the other axis.
Key ideaHorizontal and vertical motion are independent: horizontal velocity stays constant, while vertical motion has acceleration g downward. Time links the two.
Common mistakeThinking the horizontal velocity changes, or that the velocity is zero at the top (only its vertical component is).
This is one of the 39 questions in the Unit 1 quiz.
Which expression gives an object’s average velocity over a time interval?
Answer: B. Displacement ÷ time interval
Average velocity is displacement divided by the time interval. Distance divided by time gives average speed.
AP-style FRQs for this unit. Students score themselves point by point against a rubric and see a full solution.
